1. Field of the Invention
Embodiments of the present invention generally relate to an embedded optical time domain reflectometer and link continuity verification for optically amplified fiber links.
2. Description of the Related Art
Prior to optical amplification, a light signal diminished as it was transmitted through a fiber-optic transmission line. Fiber-optic transmission lines were set up to go 80 to 100 km and then convert the laser light back into an electrical signal, to be electrically amplified, then converted back to an optical signal and then sent further along the transmission line. When erbium-doped optical amplifiers were introduced, the light signal could go much further, 2 to 3000 km, without having to go through this complex, component-intensive, and therefore undesirable optical-electronic-optical conversion.
Optical amplifiers, since having been introduced, are not very expensive comparatively, are quite small, and work well, so they have been used almost exclusively in optical transmission systems since the mid-80s. However, the one disadvantage of an erbium-doped fiber amplifier approach is that the best amplifier devices cause the noise to double through the device. In less ideal, typical, amplification devices, the noise (signal) through the amplifier can be nearly quadrupled. There is a point when there is too much noise in the amplifier, which limits the maximum distance that light signals from erbium-doped fiber amplifiers can travel through glass transmission fibers,
Optical amplifiers have progressed enough in recent years so that they can be used for signal data rates of 10 Gb per second-40 Gb, and even up to 100 Gb per second. Use of erbium-doped fiber amplifiers continues, but when operating at the 100 Gb per second level, every pulse is smaller and less energetic. Each pulse sent a given rate has a certain matched/proportional energy. 100 Gb pulse rates have individual pulses that are 10 times smaller than the pulses of a 10 Gb pulse rate, so a lot less energy is put into the optical signal during signal transmission.
While clever schemes are used to get more light, basic physics dictates that there will always be less and less light energy and numbers of photons in the light signal as the pulse rate increases. As noise becomes a larger portion of the signal, the signal available for data transmission gets weaker. Performing tests in such systems with high signal rates becomes more difficult, so there has been the need to change the way in which amplifiers are used within telecommunications.
In the late 90s, a technique to improve amplification by putting a lot of laser light into the transmission fiber emerged. In that technique, rather than having the signal go along a fiber and lose power, the surplus energy in the laser light was passed to the vibrational states of the medium, the glass, to allow amplification of all-optical wavelengths. The resonantly stimulated laser light substantially maintained the power of the pulse that it encountered and provided assurance that the signal pulses did not lose as much power as similarly unstimulated pulses. This allowed data signals to go further without amplification. This technique uses a Raman amplifier, which uses Raman scattering as its light and energy producing physical process. OTDR using Raman pumps is disclosed in U.S. Pat. No. 6,850,360, which is hereby incorporated by reference herein.
The problem with a Raman amplifier is that it needs a very high power laser, which means that it is potentially very unsafe. Transmission line owners and operators do not like having a high-powered laser whose light cannot be seen by the human eye, such that when there is an open connector or a broken connector, the emitted light energy could blind somebody quite easily. The safety level standard for operation of laser apparatus and devices is defined in Standard IEC60825, which specifies the specific operational measure and safety identifier as well. The International Tele-communication Union-Telecommunication Standardization Sector (ITU-T) constitutes Standard G.664 for laser safety in communication systems. According to the Standard G.664, an optical communication apparatus should be able to automatically reduce the output power to safe power and even turn off a laser when a laser leakage occurs. The solution is expressed as an Automatic Power Reduction (APR) and an Automatic Power Shutdown (APSD) solution in G.664.
There is a need for line monitoring capabilities to be added to an optically amplified system in an efficient manner with minimal additional components. Specifically, Optical Time Domain Reflectometry (OTDR), for monitoring the attenuation along the length of a link, and Link Continuity Validation (LCV) is used for determining a valid closed bidirectional optical circuit between two nodes.